1. Field of the Invention
The present invention relates to an electrodynamic repulsive system for levitating a structure in situ or for traveling movement of the structure along a course, and, more particularly the present invention relates to such a system using transverse-flux alternating current (AC) levitation and guidance apparatus incorporating superconducting AC excitation of the primary electrical member.
2. Description of the Prior Art
U.S. Pat. No. 4,049,983 discloses a linear induction machine to provide electromagnetic levitation, through the use of a primary member having the form of four longitudinally spaced sets of linear adjacent co-planar pole faces. The core made of magnetic material, is arranged to provide paths of working flux between the pole faces in transverse planes perpendicular to the plane containing the pole faces. Polyphase excitation windings are disposed on the inner two limbs whereby no end-windings are effectively present and all windings are magnetically coupling with the core. A secondary member comprised of electrically conductive material is disposed in a confronting relation with the pole faces to produce longitudinal paths for electrical currents on each side of at least the inner pole faces of each set and transverse paths for electrical current interconnecting the longitudinal paths. When an alternating current supply is used to energize the windings of a primary member, a field of magnetomotive force operates to produce a thrust having a first component tending to force the primary member and the secondary member either apart or together and a second component tending to maintain the primary member and secondary member in alignment with one another. When the windings are energized from a plural phase alternating current supply, the resulting field of magnetomotive force travels in the longitudinal direction so that the thrust has additional components tending to cause relative longitudinal displacement between the primary member and the secondary member. To stabilize levitation, a sheet of magnetic material is arranged on the side of the electrically conductive material remote to the primary by insuring no significant quantity of magnetic flux from the primary member reaches the secondary member without first passing through the first sheet of electrically non-magnetic material. The absence of end-windings due to the unwound outer limbs of the core of the primary member is utilized to increase the power factor.
When operated for electromagnetic levitation, it is seen that the field of magnetomotive force having the component to either force apart or together the primary and secondary members lacks necessary stability in the air gap which the present invention seeks to maintain constant for attaining the necessary stability. The use of polyphase alternating current to energize this form of linear induction machine precludes use of the field of magnetomotive force for levitation. The use of only two excitation coils in the four limb core arrangement also precludes the attainment of a high power factor produced by, for example, superconductive windings due to the excessive height of the limbs and magnetization penalty associated with the extra ferromagnetic material. The outer two of four pole faces, if unexcited by alternating current, necessarily increases the size of the overall electrical machine. The space required for the electromagnetic structure is too bulky for use in a construction where the windings are liquid cooled or encased as part of a cryogenic support system for operating as a superconductor.
U.S. Pat. Nos. 3,770,995 and 3,585,423 each disclose a linear induction motor directed to producing the repulsive force as a result of flux passing in a primary and secondary magnetic structures both transversely and longitudinally to the orientation of magnetic laminations. Such induction motor is useful for propulsion of a magnetically levitated vehicle but is unsuitable for levitation and stabilization.
In U.S. Pat. No. 3,768,417 of which FIGS. 1A and 1B are taken therefrom, there is disclosed a transportation system using an electromagnetically suspended, guided and propelled vehicle A in which the vehicle is provided with a plurality of superconducting coils B distributed about a cylindrical lower surface C of the vehicle A. Adjacent coils on the vehicle are energized by direct current to produce magnetic fields D orientated with Continuously alternating polarities N-S-N-S-N-S- etc. A trough-like guideway G surrounding the lower third of the vehicle's circumference is provided with both active, current carrying conductors E and passive conductors F substantially continuous along the length of the guideway and exposed to the magnetic field of the superconductive coils B. When the vehicle speed along the guideway G reaches the predetermined minimum, the interaction between the magnetic dipoles of the vehicle coils B and the eddy currents they induce in the passive conductors F, an electrodynamic force is created to levitate the vehicle. The active conductors in the guideway are shaped in the form of a series of overlapping current loops strung axially along the guideway so as to generate when energized an alternating magnetic field which advances along the guideway and propels the vehicle at a synchronous speed by interaction with the magnetic dipoles of the vehicle's coils. The direct current excitation of the vehicle superconducting coils has the disadvantage of levitating the vehicle only after the vehicle attains the predetermined speed along the guideway. The present invention provides a system to levitate as well as guide the vehicle throughout a speed range commencing at a zero velocity and maintaining levitation and lateral guidance continuously throughout the entire operating speed of the vehicle.
In the past electrodynamic levitation systems utilize direct current excitation for not only the levitation and guidance but also the propulsion magnets. This is a severe disadvantage to the levitation and guidance of the vehicle because the magnetic forces fade out to inoperative levels at low speeds as a vehicle approaches a stand-still. Theoretical performance calculations obtained in 1976 from the Canadian Institute for Guided Ground Transportation depict the fundamental levitation lift and drag forces for a DC excited vehicle magnet suspended above a passive aluminum conductor guideway with various design thicknesses. The graph lines in FIG. 1C show the magnetic lift and drag forces as a function of speed for a specific magnet geometry used with direct current excitation. The air-core magnet represented by the calculated performance curves is 1.06 m long over the round ends and 0.3 m wide with an MMF of 400 kilo Ampere-turns (KAT), at a suspension height of 22 cm above guideway-mounted aluminum levitation strips which are 0.6 m wide.
FIG. 1C shows parametric curves for four cases of levitation strip thickness identified as follows: t.sub.1 =0.5 cm, t.sub.2 =1.0 cm, t.sub.3 =1.5 cm, t.sub.4 =2.0 cm. In FIG. 1C significant lift induced by vehicle motion generated eddy currents is not developed until a speed of 50 km per hour or greater is attained. The figure also shows the simultaneous drag versus speed characteristic for a DC excited system whereby the drag force peaks in the speed range of 30 to 100 km per hour. The electromagnetic drag is shown to peak at approximately 9 kN which imposes a restriction on the use of this system and reduces the overall efficiency of the propulsion scheme which must compensate for both the aerodynamic drag and electro-magnetic drag. The use of direct current excitation for magnets presents the problem of adapting conventional excitation schemes to contactless guideway levitation or guidance when at reduced vehicle speeds the induced guideway current is nil. At slow speeds, direct current excitation allows the magnetic flux density to remain high, but the induced voltage in the guideway electrical loop becomes too low to produce sufficient induced track currents and therefore a nil force is produced. The basic induction equation can be applied for a DC-excited system to a single electrical guideway loop enclosing the vehicle magnetic flux of one magnet at a low speed condition to estimate loop induced voltage as: EQU V=4.44 B.sub.z A f N.times.10.sup.-8 ( 1)
where:
V=induced voltage (r.m.s.) in one track loop;
4.44 is a constant;
B.sub.z =flux density (in lines/sq. in.) oriented perpendicular to vertical direction;
A=cross sectional area of track loop;
f=frequency of mechanically induced speed dependent currents; and
N=number of turns per track loop.
The relationship between the frequency "f" and pole-pitch and vehicle speed is given ##EQU1## where:
Tp=pole-pitch of track loops;
Vs=vehicle speed
Assume a vehicle speed Vs of 30 mph (11.76 m/s) according to Equation 2, the frequency "f" is found to be: ##EQU2##
Inserting this value of "f" along with the following values into Equation 1:
Bz=46,400 kl/sq. in.
N=1 turn
A=242 sq. in.
We find the induced voltage as follows: ##EQU3##
The choice of coil turns on the guideway electrical conductor is a critical parameter, but for a given volume of conductor or physical investment in guideway loop conductor material, the number of turns does not alter the basic inductance/resistance time constant (L/R) of the loop. It is economically advantageous in these systems, to have single turn loops exclusively at high speed track sections and to increase to a multi-turn (e.g. 4 turn) loop when in the designated slow-speed zones. However, there may develop a universal requirement in maglev systems, that all track sections be equally operational at high and low speeds and thus the need to have equivalent lateral stiffness and damping ratios over a very broad speed range.
The calculated loop parameters (full-scale) for a 0.319.times.0.50 m. overall dimension aluminum loop (based on formulas by F. W. Grover, Inductance Calculations, Dover Publications, 1946) are:
L=0.98 .mu..OMEGA.
R=157 .mu..OMEGA.
L/R=6.24 ms
Inductive Reactance=73.8 .mu..OMEGA.
Loop Impedance=173.5 .mu..OMEGA.
Phase Angle=25.degree.
With a 5.98 V/turn induction at 11.7 m/s vehicle speed, the induced current is therefore limited to: ##EQU4##
The maximum lateral current loading is approximately ##EQU5##
In practice, the addition of external switching devices such as thyristors to the guideway circuit loop will add at least 61 .mu..OMEGA. to the loop impedance, reducing the overall induced current to 25,500 Amps rms. This now yields a 40,000 A/m current loading for the lateral control. Thus 11.76 m/s is considered the fade-out speed on the criterion that 40,000 A/m is the lowest tolerable current loading.
Consider the case of the same vehicle magnet excitation (DC) but at a reduced speed of 6 m/s (f=6.12 Hz). The induced voltage is: ##EQU6##
The maximum lateral current loading: ##EQU7##
This restoring force for lateral guidance is too low for effectively controlling a vehicle, because to generate a 50,000N restoring force would require 50,000/13,710=23.64 sq.m. of surface area of vehicle-mounted superconducting coils which is excessive.
This level of induced voltage due to DC excitation on the vehicle magnets is at the threshold of not being able to produce sufficient circulating current in the guideway loops to produce either repulsive levitation or lateral guidance of a full sized vehicle, for example in the range 5-50 tons weight. As the speed of the vehicle is further reduced below 30 mph the induced voltage linearly decreases to 0 and consequently is wholly inoperative to generate a restoring force.
Accordingly, it is an object of the present invention to provide an electrodynamic repulsive system using superconducting alternating current excitation of a primary electrical member to levitate a structure for either static suspension or movement along a course of travel.
It is another object of the present invention to provide full vertical levitation and lateral guidance by electrodynamic forces for a moveable structure within a speed range from zero through the highest operating velocity.
It is a further object of the present invention to provide a high-power factor, lightweight design of vehicle mounted primary guidance and levitation apparatus when excited by superconducting coils with alternating current excitation.